Lewis Hamilton fastest in United States Grand Prix practice
Lewis Hamilton topped second practice at the United States Grand Prix ahead of Ferrari's Charles Leclerc.
Hamilton, who is poised to clinch his sixth world title this weekend, was 0.301 seconds quicker than Leclerc, with Red Bull's Max Verstappen third.
Sebastian Vettel was fourth, 0.357secs off team-mate Leclerc, and Valtteri Bottas fifth 0.813secs behind Hamilton.
Vettel had a spin late in the session at Turn 15. Earlier, Haas' Romain Grosjean crashed in the Esses.
Hamilton is all but certain to secure the title on Sunday. To prevent Hamilton tying up the title, Bottas must at least win the race - but even then the Briton needs only an eighth-place finish to become world champion.
And Hamilton appears to be in a very strong position heading into the weekend on a track where he has won five times in its seven appearances on the calendar.
Not only was he fastest over one lap, but his race-simulation run later in the session was extremely strong.
He and Leclerc started at a similar pace but Hamilton's tyres lasted much longer and he could keep his pace over a much greater distance, to the extent that about 10 laps into his run, Leclerc was told Hamilton's lap time, and the Monegasque replied: "No way I can do that."
Hamilton was also considerably quicker than Vettel when both were running the hard tyre on their second race-simulation run.
However, Mercedes believe that Ferrari's single-lap pace will improve on Saturday when they run their engine in its high-power modes.
Some - but not all - of Bottas' deficit was ascribed to a tow Hamilton received from Williams' Robert Kubica.
Bottas asked over the radio what explained the gap, and was told that as well as the time loss on the straights there "may be something else" to blame as well.
On the radio, his engineer did not explain what that might be.
Verstappen also looked quick at the start of a race run, but his pace also dropped off much more quickly than that of Hamilton's Mercedes.
Red Bull's Alexander Albon completed the big three in sixth, but he was just 0.075secs quicker than Toro Rosso's Pierre Gasly.
McLaren's Carlos Sainz was eighth, his team-mate Lando Norris down in 14th after having his best lap time deleted for exceeding track limits at Turn 19.
Racing Point's Lance Stroll and and Alfa Romeo's Antonio Giovinazzi completed the top 10.
Hamilton said he had been quite badly affected by the bumps on the Circuit of the Americas.
"After the first session, I was not feeling good. It was the bumpiest track I have ever been on. I had the biggest headache.
"Bumps are not such a bad thing because it adds character. So I am not a fan of completely smooth circuits but this one is massive bumps and the problem is we don't have much suspension. It's your butt on the floor and your spine takes a lot of compression, so I was feeling horrible. I had a massive headache and had to lay down. I was not feeling great. We made some changes to make it a little bit better for the second session.
"Some of them are how it was before, but today it has been pretty hardcore. I am sure they will make some changes in the future but for now we have to drive around it. If you look at the onboards, the Ferraris just sail through. I don't know if they have softer suspension. We'll just keep working on it."
Overall, though, Hamilton said he had had a positive start to the weekend.
"I've not really been looking at what the others are doing," he said. "Red Bull were looking quite good earlier and Ferrari were quite strong later. I am excited because I love this track in general. The car is feeling fairly decent but we have some work to do because there are some areas we can improve on."
Vettel said: "On one lap, we should be there or thereabouts. But on race pace we have a bit to do. We know what our weaknesses are. In qualifying we can make them but in race trim we can't."